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Abstract:

A wavelength conversion type photovoltaic cell sealing material according
to the present invention includes a first sealing layer that contains no
fluorescent substance and a second sealing layer that contains a
fluorescent substance. This wavelength conversion type photovoltaic cell
sealing material is used as one of the light transmissive layers of a
photovoltaic cell module and is disposed at a light receiving surface
side of a solar cell.

Claims:

1. A wavelength conversion type photovoltaic cell sealing material
comprising: a first sealing layer that contains no fluorescent substance;
and a second sealing layer that contains a fluorescent substance.

3. The wavelength conversion type photovoltaic cell sealing material
according to claim 1, wherein the fluorescent substance is contained in a
resin particle with a vinyl compound as a monomer compound.

4. A photovoltaic cell module comprising: a solar cell; and the
wavelength conversion type photovoltaic cell sealing material according
to claim 1, disposed at a light-receiving surface side of the solar cell.

5. The wavelength conversion type photovoltaic cell sealing material
according to claim 2, wherein the fluorescent substance is contained in a
resin particle with a vinyl compound as a monomer compound.

6. A photovoltaic cell module comprising: a solar cell; and the
wavelength conversion type photovoltaic cell sealing material according
to claim 2, disposed at a light-receiving surface side of the solar cell.

7. A photovoltaic cell module comprising: a solar cell; and the
wavelength conversion type photovoltaic cell sealing material according
to claim 3, disposed at a light-receiving surface side of the solar cell.

8. A photovoltaic cell module comprising: a solar cell; and the
wavelength conversion type photovoltaic cell sealing material according
to claim 5, disposed at a light-receiving surface side of the solar cell.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a wavelength conversion type
photovoltaic cell sealing material and a photovoltaic cell module in
which it is used. In further detail, the present invention relates to a
wavelength conversion type photovoltaic cell sealing material used in a
photovoltaic cell module which can enhance electricity generation
efficiency by converting the wavelength of light in a wavelength region
that does not contribute to electricity generation into that of light in
a wavelength region that contributes to electricity generation using a
fluorescent substance (also referred to as a light emitting material) and
to a photovoltaic cell module.

BACKGROUND ART

[0002] Silicon crystal-based photovoltaic cell modules in the related art
have such a configuration as described below. As a protective glass (also
referred to as a cover glass) on its surface, a tempered glass is used
for placing importance on shock resistance, and a pattern with recesses
and projections is applied to one side thereof by embossment in order to
improve adhesiveness with a sealing material (usually, a resin which
contains an ethylene-vinyl acetate copolymer as a main component; also
referred to as a filler).

[0003] In addition, the pattern with recesses and projections is formed on
its internal side and the surface of the photovoltaic cell module is
smooth. A shape with recesses and projections may also be applied to its
external side in order to enhance the efficiency of introducing sunlight.
In addition, a solar cell, a sealing material for protecting and sealing
a tab line, and a back film are disposed on the underside of the
protective glass.

[0004] There have been suggested a number of techniques, such as that in
Japanese Patent Laid-Open No. 2000-328053, for disposing a layer which
emits light in a wavelength region that contributes significantly to
electricity generation, in the light-receiving surface side of a
photovoltaic cell, by using a fluorescent substance and converting the
wavelength of light in the ultraviolet region or in the infrared region
that contributes less to electricity generation in the sunlight spectrum.

[0005] There have also been suggested, in Japanese Patent Laid-Open No.
2006-303033, a method for incorporating a rare earth complex, which is a
fluorescent substance, as a wavelength conversion material, into a
sealing material.

SUMMARY OF INVENTION

Technical Problem

[0006] In the above-mentioned method for converting the wavelength of
light in a wavelength region that contributes less to electricity
generation into that of light in a wavelength region that contributes
significantly to electricity generation, as described in Japanese Patent
Laid-Open No. 2006-303033, the wavelength conversion layer contains the
fluorescent substance. As the fluorescent substance, an organic
fluorescent body, an organometallic complex or an inorganic fluorescent
body, which is expensive, is used. In addition, when the wavelength
conversion layer is used as the sealing material, its film thickness of
around 600 μm is required from the viewpoint of protecting the cell.

[0007] However, when the sealing material containing the fluorescent
substance having a sufficient wavelength conversion effect is produced in
a thickness of 600 μm, the content of the fluorescent substance is
increased, so that its cost is inevitably increased and it is not always
appropriate to industrially use it.

[0008] Thus, the present invention addresses the problem of providing an
inexpensive wavelength conversion type photovoltaic cell sealing material
while maintaining or improving electricity generation efficiency when it
is applied to a photovoltaic cell module.

Solution to Problem

[0009] As a result of intensive research into the above-described problem,
the present inventors found that electricity generation efficiency
equivalent to or greater than that in the case of a large film thickness
is exhibited even in the case of a small film thickness, when the ratio
of generated electric power to incident sunlight (electricity generation
efficiency) is compared between wavelength conversion layers with small
and large film thicknesses but including fluorescent substances at the
same concentration. In view of this result, it was found that a cost
reduction can be achieved while maintaining or improving the electricity
generation efficiency when a sealing material layer on the
light-receiving side of a wavelength conversion type photovoltaic cell
module is formed so as to be divided into two layers, one layer
containing a fluorescent substance and one layer containing no
fluorescent substance.

[0010] Specifically, the present invention is as follows.

[0011] <1> A wavelength conversion type photovoltaic cell sealing
material including a first sealing layer that contains no fluorescent
substance and a second sealing layer that contains a fluorescent
substance.

[0012] <2> The wavelength conversion type photovoltaic cell sealing
material according to <1> as described above, wherein the
fluorescent substance is a europium complex.

[0013] <3> The wavelength conversion type photovoltaic cell sealing
material according to <1> or <2> as described above, wherein
the fluorescent substance is contained in a resin particle with a vinyl
compound as a monomer compound.

[0014] <4> A photovoltaic cell module including:

[0015] a solar cell; and

[0016] the wavelength conversion type photovoltaic cell sealing material
according to any one of <1> to <3> as described above,
disposed at a light-receiving surface side of the solar cell.

Advantageous Effects of Invention

[0017] In accordance with the present invention, there can be provided an
inexpensive wavelength conversion type photovoltaic cell sealing material
while maintaining or improving electricity generation efficiency when it
is applied to a photovoltaic cell module.

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 is a schematic cross-sectional view illustrating a
photovoltaic cell module according to the present invention.

[0019] FIG. 2 is a graph indicating the relationships between the film
thicknesses of wavelength conversion type photovoltaic cell sealing
materials obtained in Examples and Comparative Examples and ΔJsc.

DESCRIPTION OF EMBODIMENTS

[0020] A photovoltaic cell module according to the present invention
includes at least a solar cell and a wavelength conversion type
photovoltaic cell sealing material (wavelength conversion type
photovoltaic cell sealing sheet) disposed as one of light transmissive
layers in the light-receiving surface side of the photovoltaic cell. In
accordance with the present invention, the wavelength conversion type
photovoltaic cell sealing material (hereinafter may be simply referred to
as "sealing material") is formed by laminating a first sealing layer that
contains no fluorescent substance and a second sealing layer that
contains a fluorescent substance. The first sealing layer that contains
no fluorescent substance may be formed to include one layer or two or
more layers whereas the second sealing layer that contains the
fluorescent substance is preferably constituted by one layer from the
viewpoint of a cost or the simplification of a production process.

[0021] Since the wavelength conversion type photovoltaic cell sealing
material is constituted by the first sealing layer that contains no
fluorescent substance and the second sealing layer that contains the
fluorescent substance, the content of the fluorescent substance can be
reduced to make a production cost lower than that in the related art.
Also, electricity generation efficiency is maintained or improved due to
the wavelength conversion type photovoltaic cell sealing material having
such a two-layer structure although the content of the fluorescent
substance is reduced. The reason for this is not clear but is supposed as
described below.

[0022] When light is incident on the photovoltaic cell module, the
fluorescent substance contained in the sealing material disposed at the
light-receiving surface side absorbs light. When this occurs, it is
supposed that the absorption of light by the fluorescent substance
diminishes as the depth in the film thickness direction of the sealing
material increases. Thus, the fluorescent substance present at a deeper
portion in the film thickness direction is considered to contribute less
to wavelength conversion even if the film thickness of the sealing
material is increased. In fact, by comparing sealing materials having
fluorescent substances at the same concentration but having different
film thicknesses, it was found that electricity generation efficiency
equivalent to or greater than that in the case of a large film thickness
is exhibited even in the case of a small film thickness.

[0023] Further, the reduction in the content of the fluorescent substance
suppresses the scattering of light due to the fluorescent substance and
increases visible light transmittance. Thus, the quantity of light
arriving at the solar cell is increased, the efficiency for utilization
of light by the photovoltaic cell module is enhanced, and electricity
generation efficiency can be improved.

[0024] In the wavelength conversion type photovoltaic cell sealing
material according to the present invention, the total thickness of the
first sealing layer and the second sealing layer is preferably 10 μm
to 1000 μm from the viewpoint of a sealing effect, and more preferably
200 μm to 800 μm.

[0025] In addition, the thickness of the second sealing layer that
contains the fluorescent substance is preferably 1 μm to 800 μm,
and more preferably 10 μm to 600 μm, from the viewpoint of
wavelength conversion efficiency.

[0026] Further, the rate of the thickness of the second sealing layer that
contains the fluorescent substance to the total thickness of the first
sealing layer and the second sealing layer is preferably 0.1% to 80%, and
more preferably 1% to 50%.

[0027] The concentration of the fluorescent substance in the second
sealing layer that contains the fluorescent substance is desirably
appropriately adjusted depending on the kind of the fluorescent
substance. In general, the content of the fluorescent substance in the
second sealing layer is preferably 0.00001 to 30 parts by mass, and more
preferably 0.0001 to 10 parts by mass, based on 100 parts by mass of a
dispersion medium resin. Wavelength conversion efficiency becomes more
sufficient due to 0.0001 part by mass or more, and reduction in the
quantity of light arriving at the solar cell can be more suppressed due
to 10 parts by mass or less.

[0028] The photovoltaic cell module according to the present invention
will be explained with further reference to the drawings.

[0029] FIG. 1 is the schematic cross-sectional view of the photovoltaic
cell module according to the present invention.

[0030] In the photovoltaic cell module in FIG. 1, a protective glass (also
referred to as a cover glass) 20 is provided on the surface of the
light-receiving surface side of a solar cell 10. For the protective glass
20, a tempered glass is preferably used, without particular limitation,
in consideration of shock resistance. A pattern with recesses and
projections is preferably applied to the surface of the sealing material
side of the protective glass 20 by embossment in order to improve
adhesiveness with a sealing material (also referred to as a filler) as
described below. The light-receiving side surface of the protective glass
20 may be smooth or may be applied with a shape with recesses and
projections to enhance the efficiency of introducing sunlight.

[0031] The sealing material 30 is provided between the protective glass 20
and the solar cell 10. The sealing material 30 in FIG. 1 includes two
layers, in which the first sealing layer 32 in a light incident side is a
layer that contains no fluorescent substance and the second sealing layer
34 in a side closer to the solar cell 10 is a layer that contains a
fluorescent substance. The details of materials constituting the sealing
material 30 are described below.

[0032] The photovoltaic cell module includes a back film 40 in the back
surface side of the solar cell 10. A sealing material 36 for a back
surface, for protecting and sealing the solar cell from shock from the
back surface of the module, is provided between the back film 40 and the
solar cell 10. To the sealing material 36 for a back surface, which is
not particularly limited if being able to protect the solar cell, for
example, the same as in the first sealing layer 32 that contains no
fluorescent substance can also be applied.

[0033] Without illustration in FIG. 1, the photovoltaic cell module
according to the present invention may also further include a member,
such as an anti-reflection film, which is usually disposed in a
photovoltaic cell module.

[0035] Substances used in the wavelength conversion type photovoltaic cell
sealing material according to the present invention will be explained in
detail below.

[0036] (Fluorescent Substance)

[0037] Preferable fluorescent substances used in accordance with the
present invention include organic complexes of rare earth metals.
Especially, a europium complex or a samarium complex is preferred, and a
europium complex is more preferred.

[0038] A photovoltaic cell module having high electricity generation
efficiency can be realized by using a europium complex as a fluorescent
substance. The europium complex converts light in the ultraviolet region
into light in the red wavelength region at high wavelength conversion
efficiency and the converted light contributes to electricity generation
in a solar cell.

[0039] In a europium complex, europium (Eu) as the central element as well
as molecules that serve as ligands are needed; however, in accordance
with the present invention, the kind of the ligand is not limited, and
any molecule that forms a complex with europium may be used.

[0040] As an example of fluorescent substances formed from such a europium
complex, a rare earth complex such as Eu (TTA)3phen can be used. In
regard to a method for producing Eu (TTA)3Phen, reference can be
made to, for example, the method disclosed in Masaya Mitsuishi, Shinji
Kikuchi, Tokuji Miyashita, Yutaka Amano, J. Mater. Chem. 2003, 13,
2875-2879.

[0041] In accordance with the present invention, the ligand of the complex
is not limited, but a carboxylic acid, a nitrogen-containing organic
compound, a nitrogen-containing aromatic heterocyclic compound, a
β-diketone or a phosphine oxide is preferred as a neutral ligand.

[0042] As the ligand of the rare earth complex, a β-diketone may also
be contained, which is represented by the general formula:
R1COCHR2COR3 (wherein R1 represents an aryl group, an
alkyl group, a cycloalkyl group, a cycloalkylalkyl group, an aralkyl
group or a substitution product thereof; R2 represents a hydrogen
atom, an alkyl group, a cycloalkyl group, a cycloalkylalkyl group, an
aralkyl group or an aryl group; and R3 represents an aryl group, an
alkyl group, a cycloalkyl group, a cycloalkylalkyl group, an aralkyl
group or a substitution product thereof).

[0045] The fluorescent substance is more preferably contained in a resin
particle (also referred to as a spherical fluorescent body). A monomer
compound constituting the resin particle is not particularly limited but
is preferably a vinyl compound from the viewpoint of suppressing the
scattering of light.

[0046] In addition, as a method for making a resin particle contain the
fluorescent substance, without particular limitation, a method which is
usually used can be used. For example, it can be prepared by preparing a
mixture of a monomer compound constituting a resin particle with the
fluorescent substance and polymerizing it. Specifically, for example, a
mixture containing the fluorescent substance and a vinyl compound is
prepared, and then the vinyl compound is polymerized using a radical
polymerization initiator, so that a fluorescent material for converting
wavelengths can be constituted as a resin particle (spherical fluorescent
body) containing the fluorescent substance. In accordance with the
present invention, the fluorescent material for converting wavelengths
refers to a material in the state of being obtained by polymerizing a
vinyl compound containing a fluorescent substance.

[0047] The average particle diameter of the fluorescent material for
converting wavelengths is preferably 0.001 μm to 600 μm, more
preferably 0.005 μm to 300 μm, and further preferably 0.01 μm to
250 μm, from the viewpoint of improving efficiency for light
utilization.

[0048] The average particle diameter of the fluorescent material for
converting wavelengths can be performed by using a laser diffraction
scattering particle size distribution measuring device (e.g., LS 13320,
manufactured by Beckman Coulter, Inc.).

[0049] In accordance with the present invention, as the vinyl compound,
which is not particularly limited as long as it is a compound having at
least one ethylenically unsaturated bond, an acrylic monomer, a
methacrylic monomer, an acrylic oligomer, a methacrylic oligomer or the
like, which can become a vinyl resin, particularly an acrylic resin or a
methacrylic resin, when subjected to a polymerization reaction, may be
used without particular limitation. In accordance with the present
invention, mention is preferably made of an acrylic monomer, a
methacrylic monomer and the like.

[0050] Examples of the acrylic monomer and the methacrylic monomer include
acrylic acid, methacrylic acid and alkyl esters thereof, other vinyl
compounds that are copolymerizable with them may also be used in
combination, and the monomers may be used singly or in combination of two
or more kinds

[0051] Examples of the acrylic acid alkyl ester and the methacrylic acid
alkyl ester include acrylic acid unsubstituted alkyl esters and
methacrylic acid unsubstituted alkyl esters such as methyl acrylate,
methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate,
butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate;
dicyclopentenyl(meth)acrylate; tetrahydrofurfuryl(meth)acrylate;
benzyl(meth)acrylate; a compound obtained by reacting a polyhydric
alcohol with an α,β-unsaturated carboxylic acid (for example,
polyethylene glycol di(meth)acrylate (having a number of ethylene groups
of 2 to 14), trimethylolpropane di(meth)acrylate, trimethylolpropane
tri(meth)acrylate, trimethylolpropane ethoxytri(meth)acrylate,
trimethylolpropane propoxytri(meth)acrylate, tetramethylolmethane
tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate,
polypropylene glycol di(meth)acrylate (having a number of propylene
groups of 2 to 14), dipentaerythritol penta(meth)acrylate,
dipentaerythritol hexa(meth)acrylate, bisphenol A polyoxyethylene
di(meth)acrylate, bisphenol A dioxyethylene di(meth)acrylate, bisphenol A
trioxyethylene di(meth)acrylate, bisphenol A decaoxyethylene
di(meth)acrylate or the like); a compound obtained by adding an
α,β-unsaturated carboxylic acid to a glycidyl group-containing
compound (for example, trimethylolpropane triglycidyl ether triacrylate,
bisphenol A diglycidyl ether diacrylate or the like); an esterification
product of a polyvalent carboxylic acid (for example, phthalic anhydride)
and a substance having a hydroxyl group and an ethylenically unsaturated
group (for example, β-hydroxyethyl(meth)acrylate);
urethane(meth)acrylate (for example, a reaction product between tolylene
diisocyanate and a 2-hydroxyethyl(meth)acrylic acid ester, a reaction
product of trimethylhexamethylene diisocyanate, cyclohexanedimethanol and
a 2-hydroxyethyl(meth)acrylic acid ester or the like); an acrylic acid
substituted alkyl ester or a methacrylic acid substituted alkyl ester
having its alkyl group substituted with a hydroxyl group, an epoxy group,
a halogen group or the like; and the like.

[0052] In addition, other vinyl monomers that are copolymerizable with
acrylic acid, methacrylic acid, an acrylic acid alkyl ester or a
methacrylic acid alkyl ester include acrylamide, acrylonitrile, diacetone
acrylamide, styrene, vinyltoluene and the like. These vinyl monomers can
be used singly or in combination of two or more kinds

[0053] As the vinyl compound in accordance with the present invention, it
is preferable to use at least one selected from acrylic acid alkyl esters
and methacrylic acid alkyl esters, and it is more preferable to use at
least one selected from methyl acrylate, methyl methacrylate, ethyl
acrylate and ethyl methacrylate.

[0054] In accordance with the present invention, the radical
polymerization initiator is preferably used for polymerizing the vinyl
compound. As the radical polymerization initiator, a radical
polymerization initiator which is usually used may be used without
particular limitation. Examples preferably include a peroxide or the
like. Specifically, an organic peroxide which generates a free radical
due to heat is preferred.

[0056] The amount of the radical polymerization initiator used can be
appropriately selected depending on the kind of the vinyl compound, the
refraction index of a formed resin particle, and the like, and it is used
in a used amount that is usually employed. Specifically, for example, it
may be used in 0.1 to 15 parts by mass, and preferably used in 0.5 to 10
parts by mass, based on 100 parts by mass of the vinyl compound.

[0057] The fluorescent material for converting wavelengths in accordance
with the present invention is obtained by mixing the fluorescent
substance and vinyl compound described above, and optionally a radical
polymerization initiator such as a peroxide and the like, dissolving or
dispersing the fluorescent substance in the vinyl compound, and
polymerizing the resultant. The method of mixing is not particularly
limited, and for example, mixing may be carried out by stirring.

[0058] The content of the fluorescent substance may be preferably 0.001 to
30 parts by mass, more preferably 0.01 to 20 parts by mass, and further
preferably 0.01 to 10 parts by mass, based on 100 parts by mass of the
vinyl compound.

[0059] (Dispersion Medium Resin)

[0060] The wavelength conversion type photovoltaic cell sealing material
according to the present invention contains a dispersion medium resin in
which the fluorescent substance or the fluorescent material for
converting wavelengths is dispersed. Specific examples of the dispersion
medium resin include an acrylic resin, a polycarbonate resin, a
polystyrene resin, a polyolefin resin, a polyvinyl chloride resin, a
polyethersulfone resin, a polyarylate resin, a polyvinylacetal resin, an
epoxy resin, a silicone resin, a fluorine resin, copolymers thereof, and
the like.

[0061] The dispersion medium resins may be used singly or in combination
of two or more kinds.

[0062] Such acrylic resins as described above include a (meth)acrylic acid
ester resin and the like. Such polyolefin resins include polyethylene,
polypropylene and the like. Such polyvinylacetal resins include polyvinyl
formal, polyvinyl butyral (PVB resin), modified PVB and the like.

[0063] In addition, the (meth)acrylic acid ester resin means a resin
having a constitutional unit originating from an acrylic acid ester or a
methacrylic acid ester, and examples of the acrylic acid alkyl ester or
the methacrylic acid alkyl ester include an acrylic acid unsubstituted
alkyl ester or a methacrylic acid unsubstituted alkyl ester; an acrylic
acid substituted alkyl ester and a methacrylic acid substituted alkyl
ester having its alkyl group substituted with a hydroxyl group, an epoxy
group, a halogen group or the like; and the like.

[0064] The acrylic acid ester or the methacrylic acid ester is preferably
an alkyl ester having from 1 to 10 carbon atoms, more preferably an alkyl
ester having from 2 to 8 carbon atoms, of acrylic acid or methacrylic
acid.

[0066] The (meth)acrylic acid ester resin may be the acrylic acid ester or
the methacrylic acid ester as well as may also be a copolymer using an
unsaturated monomer copolymerizable with it.

[0067] Such unsaturated monomers as described above include unsaturated
acids such as methacrylic acid and acrylic acid; styrene,
α-methylstyrene, acrylamide, diacetone acrylamide, acrylonitrile,
methacrylonitrile, maleic anhydride, phenylmaleimide, cyclohexyl
maleimide; and the like, and two or more thereof may also be optionally
used.

[0068] These unsaturated monomers may be used singly or in combination of
two or more kinds.

[0070] Examples of the resin which is a copolymer include a (meth)acrylic
acid ester-styrene copolymer, an ethylene-vinyl acetate copolymer
(hereinafter referred to as EVA), and the like.

[0071] The dispersion medium resin is preferably EVA in terms of moisture
resistance, a cost and versatility, and is preferably a (meth)acrylic
acid ester resin in terms of durability and surface hardness. Further, a
combination of EVA and a (meth)acrylic acid ester resin is more
preferable from the viewpoint of having the advantages of both.

[0072] In EVA, the content of a vinyl acetate unit is preferably 1 to 50
mass %, and preferably 3 to 35 mass % in terms of homogeneous
dispersibility of the fluorescent substance in the sealing material.

[0073] From the viewpoint of forming the sheet, the content of the vinyl
acetate unit in EVA is preferably 10 to 50 mass %, and more preferably 20
to 35 mass %.

[0075] When EVA is used together with methyl methacrylate, the content of
EVA is preferably 50 parts by mass or more, and more preferably 70 parts
by mass or more, based on the total amount of 100 parts by mass of EVA
and methyl methacrylate.

[0076] Further, the dispersion medium resin may also be a resin having a
crosslinked structure, to which a crosslinkable monomer is added.

[0077] Examples of the crosslinkable monomer may include a compound
obtained by reacting a polyhydric alcohol with an
α,β-unsaturated carboxylic acid (for example, polyethylene
glycol di(meth)acrylate (having a number of ethylene group of 2 to 14),
trimethylolpropane di(meth)acrylate, trimethylolpropane
tri(meth)acrylate, trimethylolpropane ethoxytri(meth)acrylate,
trimethylolpropane propoxytri(meth)acrylate, tetramethylolmethane
tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate,
polypropylene glycol di(meth)acrylate (having a number of propylene
groups of 2 to 14), dipentaerythritol penta(meth)acrylate,
dipentaerythritol hexa(meth)acrylate, bisphenol A polyoxyethylene
di(meth)acrylate, bisphenol A dioxyethylene di(meth)acrylate, bisphenol A
trioxyethylene di(meth)acrylate, bisphenol A decaoxyethylene
di(meth)acrylate, or the like); a compound obtained by adding an
α,β-unsaturated carboxylic acid to a glycidyl group-containing
compound (for example, trimethylolpropane triglycidyl ether triacrylate,
bisphenol A diglycidyl ether diacrylate, or the like); an esterification
product of a polyvalent carboxylic acid (for example, phthalic anhydride)
and a substance having a hydroxyl group and an ethylenically unsaturated
group (for example, β-hydroxyethyl(meth)acrylate); a urethane
(meth)acrylate (for example, a reaction product between tolylene
diisocyanate and 2-hydroxyethyl(meth)acrylic acid ester, a reaction
product of trimethylhexamethylene diisocyanate, cyclohexanedimethanol and
2-hydroxyethyl(meth)acrylic acid ester, or the like); and the like.

[0079] The above-described crosslinkable monomers are used singly or in
combination of two or more kinds.

[0080] Alternatively, the dispersion medium resin can be made to have a
crosslinked structure by adding a radical polymerization initiator to the
above-described monomer and polymerizing the resultant by heating or
light irradiation.

[0081] As the radical polymerization initiator, a radical polymerization
initiator which is usually used may be used without particular
limitation. Examples include the above-mentioned peroxides and the like.

[0082] The weight average molecular weight of the dispersion medium resin
is preferably 10,000 to 100,000, and more preferably 10,000 to 50,000,
from the viewpoint of flowability.

[0083] The wavelength conversion type photovoltaic cell sealing material
according to the present invention may also optionally contain an
ultraviolet absorbing agent, a coupling agent, a plasticizer, a flame
retardant, an antioxidant, a light stabilizer, a rust-preventive agent, a
processing aid or the like, as well as those described above.

[0084] The wavelength conversion type photovoltaic cell sealing material
according to the present invention can be produced by using a known
technology. For example, a method for molding, into sheet form, a
composition, in which at least the fluorescent substance or the
fluorescent material for converting wavelengths (spherical fluorescent
body), and the dispersion medium resin, and further optionally another
additive are melt-kneaded, a method for making the dispersion medium
resin into varnish, to which the fluorescent substance or the fluorescent
material for converting wavelengths (spherical fluorescent body) is
added, followed by molding the resultant into sheet form, and removing a
solvent, or the like can be utilized.

[0085] Specifically, the wavelength conversion type photovoltaic cell
sealing material is obtained by, for example, making two mold release
sheets face each other via a spacer, applying the melt-kneaded
composition into a gap formed between the two mold release sheets,
heat-pressing it from both sides to form a second sealing layer, further
performing the same method but forming a first sealing layer that
contains no fluorescent substance, laminating the second sealing layer
and the first sealing layer, sandwiching them between the mold release
sheets, and heat-pressing them from both sides.

[0086] <Photovoltaic Cell Module>

[0087] In accordance with the present invention, a photovoltaic cell
module is constituted by necessary members such as an anti-reflection
film (not illustrated), a protective glass 20, the wavelength conversion
type photovoltaic cell sealing material 30 mentioned above, a solar cell
10, a sealing material 36 for a back surface, a back film 40, a cell
electrode (not illustrated) and a tab line (not illustrated).

[0088] The anti-reflection film (not illustrated), the protective glass
20, and the wavelength conversion type photovoltaic cell sealing material
30 according to the present invention, which are present closer to a
light incident side than the solar cell 10 among these members, are
disposed in this order.

[0089] In accordance with the photovoltaic cell module according to the
present invention, in order to ensure that external light that enters
from any angle is efficiently introduced into a solar cell with reduced
reflection loss, it is preferable that the refraction index of the
wavelength conversion type photovoltaic cell sealing material 30 is
higher than the refraction indices of the light transmissive layers that
are disposed closer to the light incident side than the wavelength
conversion type photovoltaic cell sealing material 30, that is, the
anti-reflection film, the protective glass 20 and the like; and is lower
than the refraction indices of the light transmissive layers that are
disposed closer to the opposite side from the light incident side than
the wavelength conversion type photovoltaic cell sealing material 30,
that is, a cell anti-reflection film (not illustrated), the solar cell 10
containing Si and the like.

[0090] That is, in accordance with the photovoltaic cell module according
to the present invention, the refraction index of the layer disposed at
the side closer to the solar cell 10 is desirably equivalent to or higher
than the refraction index of the adjacent layer disposed at the light
incident side, in the layers disposed on the solar cell 10 and closer to
the light incident side than the solar cell 10 (such as the protective
glass 20 and the anti-reflection film (not illustrated) disposed closer
to the light incident side than the protective glass 20).

[0091] Specifically, assuming that the solar cell 10 and the layers
disposed closer to the light incident side than the solar cell 10 include
m layers (m is 2 or more) and the respective refraction indices of the m
layers are n1, n2, . . . , nm-1, nm in order from the
light incident side, it is desirable that n1≦n2≦
. . . ≦nm-1≦nm be satisfied. Since the wavelength
conversion type photovoltaic cell sealing material 30 according to the
present invention is configured by two or more sealing layers, the
refraction indices of the two or more sealing layers also preferably
satisfy this relationship.

[0092] Specifically, the light transmissive layers placed closer to the
light incident side than the wavelength conversion type photovoltaic cell
sealing material 30, that is, the anti-reflection film which has a
refraction index of 1.25 to 1.45 and the protective glass 20 which has a
refraction index of usually around 1.45 to 1.55 are used. The light
transmissive layers placed in the anti-light incident side of the
wavelength conversion type photovoltaic cell sealing material, that is,
the cell anti-reflection film of the solar cell which has a refraction
index of usually around 1.9 to 2.1 and the Si layer or the like
constituting the solar cell which has a refraction index of usually
around 3.3 to 3.4 are used.

[0093] The preferred refraction indices of other layers in the light
transmissive layers are as described below. For example, assuming that
three layers from the light incident side of the light transmissive
layers are layer a, layer b and layer c, it is preferable that the
refraction indices na, nb and nc of the respective layers meet or be
similar to the following equation (1):

nb=(nanc)0.5

[0094] The disclosure of Japanese Application No. 2010-120647 is
incorporated herein by reference in its entirety.

[0095] All literatures, patent applications and technical standards
described in the present specification are herein incorporated by
reference to the same extent as if each individual literature, patent
application and technical standard was specifically and individually
indicated as being incorporated by reference.

EXAMPLES

[0096] The present invention will be described in more detail below with
reference to Examples, but the present invention is not limited to these
Examples.

Example 1

<Synthesis of Fluorescent Substance>

[0097] In 7 ml of ethanol, 200 mg of
4,4,4-trifluoro-1-(thienyl)-1,3-butanedione (TTA) was dissolved, and 1.1
ml of 1M sodium hydroxide was added thereto and mixed. To the foregoing
mixed solution, 6.2 mg of 1,10-phenanthroline dissolved in 7 ml of
ethanol was added, and the resultant was stirred for one hour, followed
by adding 3.5 ml of an aqueous solution containing 103 mg of
EuCl3.6H2O to obtain a precipitate. It was filtered off, washed
with ethanol, and dried to obtain a fluorescent substance
Eu(TTA)3Phen.

[0099] Using 0.3 part by mass of Eu(TTA)3Phen obtained above as a
fluorescent substance, 60 parts by mass of methyl methacrylate as a vinyl
compound, and 0.012 part by mass of n-octanethiol as a chain transfer
agent, these are mixed and stirred to prepare a monomer mixture liquid.
In addition, 300 parts by mass of ion-exchanged water and 3.65 parts by
mass of sodium alkylbenzene sulfonate, G-15, manufactured by Kao
Corporation, as a surfactant, were added, the previously-mentioned
monomer mixture liquid was added thereto, the resultant was kept at
60° C. while stirring it using a flask with a reflux pipe under a
nitrogen stream, 0.03 part by mass of potassium persulfate as a radical
polymerization initiator was added, emulsion polymerization was carried
out for 4 hours, and its temperature was finally increased to 90°
C. to complete the polymerization reaction.

[0100] The thus obtained fluorescent material for converting wavelengths
became particulate matter having a primary particle diameter of around
100 nm, which was appropriately subjected to post-treatment with
isopropyl alcohol or the like, filtered off, dried, and appropriately
sieved to obtain a particulate fluorescent material for converting
wavelengths (spherical fluorescent body).

[0101] <Preparation of Resin Composition for Converting Wavelengths>

[0102] In a roll mill at 90° C., 100 parts by mass of an
ethylene-vinyl acetate resin (EVA): NM30PW manufactured by Tosoh
Corporation as a transparent dispersion medium resin, 1.5 parts by mass
of a peroxide thermal radical polymerization initiator (in the present
example, also functions as a crosslinking agent): Luperox 101
manufactured by Arkema Yoshitomi, Ltd., 0.5 part by mass of a silane
coupling agent: SZ6030 manufactured by Dow Corning Toray Co., Ltd., and
0.01 part by mass of a fluorescent substance [which was added in the form
of a fluorescent material for converting wavelengths (spherical
fluorescent body). 1 part by mass of the fluorescent material for
converting wavelengths was equivalent to 0.005 part by mass in terms of
the fluorescent substance] were knead to obtain a resin composition for
converting wavelengths.

[0103] <Production of First Sealing Sheet Containing No Fluorescent
Substance>

[0104] A resin composition was prepared in the same manner as in the
preparation of the resin composition for converting wavelengths as
described above except that any fluorescent material for converting
wavelengths (spherical fluorescent body) was not added. About 6 g of this
resin composition was sandwiched between mold release sheets, and a first
sealing sheet containing no fluorescent substance, with a thickness of
about 328 μm, was produced by a pressing machine having a hot plate
adjusted to 90° C. using a stainless spacer.

[0106] A second sealing sheet containing a fluorescent substance, with a
thickness of about 272 μm, was obtained with the resin composition for
converting wavelengths, obtained above, in the same manner as in the
production of the above-described first sealing sheet except that the
thickness of a spacer was changed.

[0108] The first sealing sheet and the second sealing sheet described
above were sandwiched between mold release sheets to obtain a wavelength
conversion type photovoltaic cell sealing material having a two-layer
structure by a pressing machine having a hot plate adjusted to 90°
C. using a stainless spacer. The obtained wavelength conversion type
photovoltaic cell sealing material has a thickness of 600 μm.

[0109] <Production of Photovoltaic Cell Sealing Sheet for Back
Surface>

[0110] A photovoltaic cell sealing sheet for a back surface, with the same
composition as that of the first sealing sheet described above, was
produced by the same method except that it was adjusted to have a
thickness of 600 μm.

[0112] The wavelength conversion type photovoltaic cell sealing material
described above was mounted on protective glass (tempered glass;
manufactured by Asahi Glass Co., Ltd.) so that the first sealing sheet
for converting wavelengths containing no fluorescent material (spherical
fluorescent body) was brought into contact with the tempered glass, a
solar cell designed so as to be able to supply electromotive force to the
exterior was mounted thereon, the photovoltaic cell sealing sheet for a
back surface and a back film (PET film; trade name: A-4300, manufactured
by Toyobo Co., Ltd.) were further mounted thereon, and lamination was
carried out under the conditions of a hot plate at 150° C., a
vacuum for 10 minutes, and pressurization for 15 minutes using a vacuum
pressure laminator for a photovoltaic cell (NPC Incorporated,
LM-50×50-S) to produce a photovoltaic cell module in Example 1.

[0113] The solar cell designed to be able to supply electromotive force to
the exterior was a solar cell in which an electrically-conductive film
CF-105 for a photovoltaic cell, manufactured by Hitachi Chemical Company,
Ltd., was used, two tab lines (0.14 mm in thickness, 2 mm in width,
galvanized) were connected to the front and two tab lines to the back by
a specialized application device, and each of these at the front and back
was further configured as an external supply line using a horizontal tab
line (A-TPS 0.23×6.0, manufactured by Hitachi Cable, Ltd.). In
addition, in the solar cell designed to be able to supply electromotive
force to the exterior, photovoltaic cell I-V characteristics were
obtained prior to modularization using a solar simulator WXS-155S-10,
AM1.5G, manufactured by Wacom Electric Co., Ltd., and an I-V curve tracer
for a solar simulator, MP-160, manufactured by EKO Instruments Co., Ltd.
Jsc (short-circuit current density), measured and obtained according to
JIS-C-8914, was regarded as Jsc (cell).

[0115] A wavelength conversion type photovoltaic cell sealing material in
Example 2 was produced in the same manner as in the production of the
first and second sealing sheets in Example 1 except that the thicknesses
were changed as listed in Table 1.

[0117] A wavelength conversion type photovoltaic cell module in Example 2
was produced in the same manner as in Example 1 except that a change into
the above-described wavelength conversion type photovoltaic cell sealing
material in Examples 2 was made.

[0119] Wavelength conversion type photovoltaic cell sealing materials in
Comparative Example 1 and Comparative Example 2 were produced in the same
manner as in the production of the second sealing sheet in Example 1
except that the thicknesses were changed as listed in Table 1.

[0121] The above-described wavelength conversion type photovoltaic cell
sealing material in Comparative Example 1 or Comparative Example 2 was
mounted on a tempered glass (manufactured by Asahi Glass Co., Ltd.) as a
protective glass, a solar cell designed to be able to take an
electromotive force to the outside was upward mounted thereon, the
photovoltaic cell sealing sheet for a back surface and a PET film (trade
name: A-4300, manufactured by Toyobo Co., Ltd.) as a back film were
further mounted, and lamination was carried out on the conditions of a
hot plate at 150° C., a vacuum for 10 minutes, and pressurization
for 15 minutes using a vacuum pressure laminator for a photovoltaic cell
(NPC Incorporated, LM-50×50-S) to produce photovoltaic cell modules
in Comparative Example 1 and Comparative Example 2.

[0122] [Evaluation of Photovoltaic Cell Module]

[0123] In the wavelength conversion type photovoltaic cell modules
produced as described above, photovoltaic cell I-V characteristics were
obtained using a solar simulator WXS-155S-10, AM1.5G, manufactured by
Wacom Electric Co., Ltd., and an I-V curve tracer for a solar simulator,
MP-160, manufactured by EKO Instruments Co., Ltd. and were regarded as
Jsc (module) measured and obtained according to JIS-C-8914. Using this
value and premeasured Jsc (cell), ΔJsc was calculated from the
following equation:

ΔJsc=Jsc(module)-Jsc(cell)

[0124] The obtained results are summarized in Table 1, and the
relationships between the film thicknesses of the wavelength conversion
type photovoltaic cell sealing sheets containing the fluorescent
materials for converting wavelengths (spherical fluorescent bodies) and
ΔJsc are summarized in FIG. 2.

[0125] As seen in Table 1 and FIG. 2, it was demonstrated that the
wavelength conversion type photovoltaic cell sealing material including
the two layers containing and not containing the fluorescent substance
had a greater wavelength conversion effect, even if the film thickness of
the sheet containing the fluorescent substance was 300 μm or less,
than that of the sheet in which the wavelength conversion type
photovoltaic cell sealing material included the one layer containing the
fluorescent substance and its film thickness was 590 μm. That is, it
was revealed that the amount of the fluorescent substance used was
reduced to half or less and conversion efficiency was improved.

[0126] In a 200 ml screw pipe, 0.05 g of the fluorescent substance
Eu(TTA)3Phen obtained above, 95 g of methyl methacrylate, 5 g of
ethylene glycol dimethacrylate, and 0.5 g of
2,2'-azobis(2,4-dimethylvaleronitrile) as a thermal radical initiator
were each metered and put, and were stirred and mixed using an ultrasonic
washer and a mix rotor. To a separable flask with a cooling pipe, 500 g
of ion-exchanged water and 59.1 g of 1.69% polyvinyl alcohol solution as
a surfactant were added and stirred. The previously prepared mixture
liquid of methyl methacrylate with ethylene glycol dimethacrylate was
added thereto, and the resultant was heated to 50° C. and reacted
for 4 hours while being stirred at 350 rpm. When the particle diameter of
this suspension was measured using Beckman Coulter LS13320 (particle size
distribution measuring device by high-resolution type laser diffraction
scattering method) manufactured by Beckman Coulter, Inc., the volume mean
diameter was 104 μm. A precipitate was filtered off, washed with
ion-exchanged water, and dried at 60° C. to obtain a fluorescent
material 2 for converting wavelengths (spherical fluorescent body) by
suspension polymerization.

[0128] In a roll mill at 90° C., 100 parts by mass of an
ethylene-vinyl acetate resin (EVA): NM30PW manufactured by Tosoh
Corporation as a transparent dispersion medium resin, 1.5 parts by mass
of a peroxide thermal radical polymerization initiator (in the present
example, also functions as a crosslinking agent): Luperox 101
manufactured by Arkema Yoshitomi, Ltd., 0.5 part by mass of a silane
coupling agent: SZ6030 manufactured by Dow Corning Toray Co., Ltd., and 1
part by mass of the fluorescent material 2 for converting wavelengths
(spherical fluorescent body) obtained above and subjected to the
polymerization (1 part by mass of the fluorescent material for converting
wavelengths was equivalent to 0.0005 part by mass in terms of the
concentration of the fluorescent substance) were knead to obtain a resin
composition 2 for converting wavelengths.

[0130] A wavelength conversion type photovoltaic cell sealing material in
Example 3 was produced in the same manner as in the production of the
first sealing sheet in Example 1 except that a change into the
above-described resin composition 2 for converting wavelengths was made.

[0132] A wavelength conversion type photovoltaic cell module in Example 3
was produced in the same manner as in Example 1 except that a change into
the above-described wavelength conversion type photovoltaic cell sealing
material in Example 3 was made.

[0133] [Evaluation of Photovoltaic Cell Module]

[0134] When the evaluation of the wavelength conversion type photovoltaic
cell module in Example 3 was carried out by the above-described method,
ΔJsc was 0.73 mA/cm2, so that it was found to be superior in
conversion efficiency to that in Example 1.